Solvation dynamics of nile blue in ethanol confined in porous sol-gel glasses

We report on solvation dynamics measurements of the chromophore nile blue i n ethanol confined to sol-gel glasses with 50 Angstrom and 75 Angstrom aver age pore size and compare them with the dynamics of the respective bulk sol ution. Both the amplitude of the dynamic Stokes shift as well as the dynami cs of the solvation process are drastically changed upon confinement. In bo th confined solutions the dynamic Stokes shift is reduced by a factor of ab out 2. As the large majority of the chromophores is adsorbed at the inner p ore surfaces the solute molecules interact with only a "half-space" of solv ent molecules. In a first approximation this decreases the stabilization en ergy by a factor of 2. The solvation dynamics in the confined solutions sho w nonexponential behavior comparable to the bulk. However, the whole solvat ion process slows down and the single decay time constants characterizing i t increase with decreasing pore size of the sol-gel glass. We have introduc ed two phenomenological models to rationalize this behavior. The enhanced p olarization field model takes into account the confinement and predicts a s trengthening of the solvent's polarization field in an extended solvation c age induced by the electrical field of the chromophore. The steric hindranc e model focuses on the interaction of the liquid with the surface reducing molecular mobility resulting in longer relaxation times. Probably both effe cts are of relevance in the confined solutions investigated. Confinement af fects the steady-state spectra as well leading to a redshift of the absorpt ion and a blueshift of the fluorescence. Additional data on the reorientati onal dynamics of the chromophore are consistent with the fact that the mole cules are predominantly adsorbed at the pore surfaces. (C) 2001 American In stitute of Physics.